She then joined the Secret Intelligence Service (MI6), but when Kim Philby, later to be exposed as a double agent, became her boss he reduced her investigative work because he feared she might uncover his treachery.

[3] In her spare time she trained to be a barrister, becoming the fifth woman to be admitted to Gray's Inn, and, after obtaining first-class exam results, was called to the bar in 1924.

[9][10] In 1937 Roger Hollis applied to join MI5 and Kell asked Sissmore to make an informal assessment of him which she did at her tennis club, also involving the then still junior Dick White.

[5][14] Krivitsky had been a Soviet agent in continental Europe who had defected to the West in 1937 and whose revelations in the United States in September 1939 had created a press sensation.

In particular from a British point of view he revealed that two Soviet agents were currently working in Britain – the one whose surname he was able to give was quickly identified, tried and convicted of spying.

[15][16] Archer's report (a "masterly analysis" according to Nigel West) was 85 pages long and much of it subsequently became incorporated in an MI5 overall review of Soviet intelligence activity.

[17] Krivitsky's disclosures might have made it possible to work out that both Donald Maclean and Kim Philby were Soviet agents but this opportunity was missed.

[22] Archer's debriefing completely transformed the understanding of the top echelons of MI5 about current Soviet espionage activity in Britain – they now realised it was extensive whereas only a year earlier it had wrongly been thought non-existent.

[25] At a top-level meeting at that venue in November Archer criticised Brigadier Harker, recently appointed acting director of MI5, for incompetence.

[5][26][27] Concerning this Guy Liddell, director of counter-espionage, wrote of Harker "but for his incompetence, the situation would never have arisen" but he also thought that Archer had "unfortunately gone too far".

It was she who had interrogated General Krivitsky, the Red Army intelligence officer who defected to the West in 1937, only to kill himself a few years later in the United States – a disillusioned man.

And here she was pluncked down in my midst!Philby (who had himself been "the young English journalist") made efforts to guard against the threat of her uncovering his background and his current role as a Soviet agent.

[38][39] Igor Gouzenko, a cipher clerk in Soviet embassy in Ottawa, defected in 1945 with a mass of documents, one of which led to the arrest of Alan Nunn May for spying.

[note 5] Agreeing this, MI5 sent Hollis and yet it was Philby at SIS who was apprised of progress and who altered, delayed and omitted messages before passing them on to MI5.

In 1949, in the aftermath of the test explosion of the Soviet Union's first atomic bomb, new United States Venona decrypts led the FBI to conclude that Fuchs had been a spy on the Manhattan Project and, although subsequent MI5 surveillance produced little evidence of his still being a spy, Fuchs confessed to espionage in both countries and he was sentenced to 14 years' imprisonment.

[45] Archer also recommended that Rudolf Peierls should not be allowed to continue as a consultant to Harwell but he went on to have a highly distinguished scientific career.

[47] Presented with MI5's evidence and under pressure from Washington, the head of SIS still did not doubt Philby's loyalty but asked him to retire with a golden handshake.

[48][49][50][note 8] In 1952, while examining the papers left in Burgess's flat, Archer found documents describing secret meetings that were then discovered to have been written by John Cairncross.

It was sadly ironic that Wright and Martin, the most damaging conspiracy theorists in the history of the Security Service, should later try and persuade themselves that the unidentified agent was Hollis himself.